Current immune suppression protocols have greatly prolonged the life span of the average transplant and transplant recipient, alloantibody mediated rejection remains a limiting factor in long-term transplant survival. Animal models have shown platelet interactions with an inflamed endothelium amplify the immune response, increase leukocyte adhesion, and exacerbate atherosclerosis. However, the role of platelets in transplant rejection is not well understood. The long-term objective of our proposal is to define mechanisms of platelet mediated transplant rejection. Specific Aim #1: To demonstrate that platelets promote leukocyte trafficking and endothelial cell inflammation in immune responses to transplants. Leukocyte and endothelial cell interactions are pivotal for both the innate and acquired immune responses. We will demonstrate the role of platelet derived adhesion molecules in promoting leukocyte localization and trafficking in alloimmune responses to transplants. Specific Aim #2: To demonstrate that platelet derived chemokines promote an innate immune response in transplantation. Platelets also secrete molecules, including chemokines, that have immuno-regulatory functions and promote leukocyte trafficking. We will demonstrate that platelet derived chemokines increase neutrophil (PMN) and monocyte activation and trafficking as part of innate alloimmune responses. Specific Aim #3: To demonstrate that platelet derived chemokines promote an acquired immune response in transplant rejection. PUBLIC HEALTH RELEVANCE: To further explore the important role of platelet derived chemokines in transplant rejection we will demonstrate that platelets also increase transplant directed T-cell activation and recruitment. In 2005 alone almost 27,000 Americans received an organ transplant. Antibody mediated transplant rejection remains a difficult to control cause of transplant loss and its mechanisms are poorly understood.